Engine/Transmission (1994 - 1998) Water Temperature Cycling

[UKRugbyman77]

I have a question on a 97 model. I have a buddy that says when he starts off, the water temp guage starts off rock bottom (which is normal). It then proceeds to climb (normally) until it gets to about 200 degrees, then goes back to almost rock bottom immediately. Then it proceeds to rise again gradually until it hits ~200 deg, then back down again. He said it cycles like this constantly and it take about 10-15 minutes for one of these cycles to complete. He's already replaced the thermostat (190) but that didn't help. He's looking at getting a new sending unit now. Can anybody relate to this problem - help



Also, does anybody know if and where the bleeder valve is on this model. Might be air pockets doing this??

Much Obliged. .

 

[scaryfast]

I think your going to hear that Dodge considers the temp cycling normal for the Cummins engine. There are some recommended thermostat changes and I've tryed them all, nothing helped.

Scaryfast

 

[Joe G.]

It's caused by the big cooling system these trucks have. There are stats that help the cycling but I not sure that they don't get in the way of cooling when you really need it. If you will look at the range of the cycle you will see that the bottom temp is usually about 150 and the top is usually about 180 or so. So what you are seeing is a temp variation of between 30 to 40 degrees in coolant temp. Not necessarily seeing that fluctuation in actual iron temp. Compared to the temp variation in the iron close to the combustion chambers the coolant variation you are seeing on the temp gauge is nothing. If my quit cycling I would start to worry that something was going wrong with my cooling system. The stat in my '95 is the original. I have 209K miles on it.

 

[illflem]

Nothing is wrong with the sender

Cummins has a "new improved" tsat that is supposed to stop the cycleing, I've found all it does is make the truck run too cold. I went with a 180° Stant tsat from the auto parts, it still cycles but not enough to be of concern.

 

[Ben Rumson]

No problem

My '95 with 156k has done this from day one. Mine fluctuates between 150 and 190 constantly.



I concur with Joe G. - Iron temp does not fluctuate this rapidly.



The whole scenario is played out over and over again, mile after mile as a result of the great cooling ability of these trucks.



I would only replace the t-stat if the gauge starts getting over 200 and stays there OR if it bottoms out and stays there.



Otherwise, you'll just be replacing a perfectly good t-stat.



Chin up.



- Ben

 

[UKRugbyman77]

Thanks for the input, however, his just STARTED doing this all of a sudden, and he's had it for a number of years. He says he knows the exact time it happened (the first actual lengthy trip the truck went on - from Maryland to TN) Why the sudden change?

I appreciate all the input tho.

???

 

[Ben Rumson]

well then...

Something had to have happened... . but what?



It's possible that the some gasket material has been caught in the t-stat, but I can't see where that would suddenly cause the fluctuation.



Perhaps there's an obstruction somewhere in the cooling system and it needs a good flush... . stabbing in the dark here.



Maybe it was just breaking itself in and now it's going to act like the rest of ours.



Although I don't have an answer as to "why" I still don't don't believe it's a problem.



Anyone else?

 

[Berrigan]

My first gen truck did that fluctuation thing to but my 98 does not.



I always noticed it more in the winter. During the summer I think the h2o stuck in the radiator during closed stat operation does not get as cool. So when the stat opens the sending unit does not see a rush of cold water rather a rush of warm (AC condenser) in front of it, water.



You asked about venting. The first gen trucks had jiggle pins built into the t-stat. The second gens have a remote mounted jiggle pin (vent) that is suppose to operate automatically every time the water pump stops pumping. It will let any air vent past and thru the little u shaped hose to the radiator where it should get vented out thru the rad cap on the next warm up cycle. lol!

 

[Joe G.]

UKRugbyman77,



Your friend should consider this: a temp cycle is normal, no cycle is NOT normal. He may have had a piece of the gasket stuck in the stat and it came loose when he worked it hard.

 

[UKRugbyman77]

Will most definitley pass it on thanks fellers.

 

[Deezul1]

My 96 and 97 do it and I replaced a bad stat in the 97 with the new and improved stat from Cummins that was supposed to stop it from cycling and as mentioned above it ran too cool so replaced it with a NAPA stat which still runs a little cool but doesnt cycle much at all and is 1/3 the price of the Cummins stat.

 

[drees1]

Mine cycled from the time I bought it with 124,000 miles on it. I put in the revised thermostat that Cummins has and it does run a little cooler, but the problem is resolved. It will sometims start out cycling, but eventually evens out after 3 or 4 hard cycles. Ambients around 55 - 65 deg seem the worst.



Although the Cummins makes it through the warranty period and beyond with this behavior, it is not the best thing you can do to your engine.



The response rate of the thermostat, bypass bleed, radiator capacity, cooling flow, bypass flow and waterpump curve are all part of the equation that creates this instability. The only control element in the system is the thermostat. So that is all that can be changed to resolve the problem.



I have designed diesels at CAT and Cummins, and a thermal cycle of the cooling system is a standard test for headgasket durability. A temp fluxuation from 200 to 130 every 2 minutes (as in my truck) is plenty to eventually induce a head gasket leak on many engines. The iron responds to the temp change, otherwise it would not cycle so fast. Dare to try that with an Aluminum head?



I acccept the argument that it is "OK" based on the fact that this particular engine tolerates it. But it is not a good thing in general.



To resolve the problem, the thermostat needs to be faster or slower response, or be designed to have a smaller rate of flow so it has more time to react and does not dump all the water to the rad in a big slug. What has been done on the revised stat is the flow rate through the stat has been lowered upon initial opening as a function of temp. So to avoid an over heat problem from slower response, the temp setting is dropped a little to start the flow to the rad sooner. In an unloaded condition, this results an equalibrium point established at a lower temp, thus we see lower temps with the new stat unloaded.

 

[JJW_ND]

Interesting......

My truck cycled for the first 100K +. Changed several stats, winter fronts, etc. I pulled allot and when towing heavy I had to pull winter front. Even pulling hard in temps below -10 the stat would cycle. With winter front (had about a 10" square openning) would get to warm. I did everything I could the keep enough heat to keep it steady!



At about 120K I get a very small seep by the Tstat housing. I finally changed the headgasket at 220K. Leak was very slow about a gallon every 20K miles.



I never thought it was good to see that big swing in coolant temps. When towing it was a pretty fast cycle.



I also notice the newer stat runs allittle cooler. With no engine fan and a winter front it actually seems to run slightly warmer. Always steady though!



Might be something to the Tstat cycle and headgasket???



jjw

ND

 

[Bseg]

My stock tst went bad at around 50k miles. I bought one at Napa and it cycled like the stock one did when the truck was new and now it has around 30k miles on it and it doesn't cycle as much. Maybe as time goes on the tst gets warn out and the cycling gets to be less, just a possible idea?

 

[Matt42]

After six years and 45,000 miles (please do not laugh--loudly), the temperature fluctuations on mine quieted down and the needle stays pretty steady. The indicated temperature will be higher or lower depending on whether I am towing or not.



This is with the OEM thermostat still installed, and coolant changed from original to Dex-cool meeting ASTM 4895.



Should I be concerned?

 

[Joe G.]

I can't help but wonder if this cycling is so bad why do the engines in these trucks last a million miles?

 

[illflem]

I'm with you Joe, I don't concern myself with it.

 

[John Berger]

I know this is the 12V forum, but thought this relevant enough to say here. I replaced my stock T-stat recently with the 180 degree T-stat from Cummins. It cycles much faster than the original T-stat did. I can't remember whether this occurred when the truck was new. Based upon the water temp gauge, I would say that the temperature varies from 160-175 degrees. Even in cold temperatures like we're seeing in this part of Georgia now, there is a noticeable difference in temperatures with the new T-stat as compared to the original. The original did not fluctuate as much. It will be interesting to see if there is a leveling off after time and mileage. I haven't had a chance to tow yet, so that will be interesting to observe as well.

 

[Slow Six]

t-stast cycling

I have had a lot of cycling on my 94. I am presently using a 195 degree super stat from Napa. I use the 195 year around including heavy towing in the summer. I use a weather front in winter to help keep things warm. I have had to go through a few statS to find one that won't cycle a lot, but Napa warrants them. I have found that performance and mileage are better with the 195 in winter. In cab heat is also improved especially in -40 weather.

 

[JJW_ND]

Part Number

You have the NAPA number for this 195 degree stat? or is it the same number for the 180 stat that is often talked about?



While I like the non flutuating Cummins Stat, I really wished it ran warmer. I know better MPG happen with a warmer coolant temp.



jjw

ND